In recent years, because of having a surface area larger than that of granular activated carbon, activated carbon fibers processed into textile fabric, felt, mat, etc. have come to be used as adsorbents or filters for solvent recovery device or air cleaning system, etc., and have attracted attention.
It is known that such activated carbon fibers may be prepared from such starting materials, for example, as polyacrylonitrile fibers, phenolic resin fibers, cellulosic fibers and pitch fibers. As regards the polyacrylonitrile activated carbon fibers, there has been proposed a method for the preparation thereof, for example, by oxidation treatment of acrylonitrile fibers containing iron compounds at a specific temperature, followed by activation treatment (Japanese Patent Publication No. 53294/1988). In this method, however, there are involved such problems that though the activated carbon fibers obtained have a relatively high strength such as 20-50 kg/mm.sup.2, the specific surface area thereof is as small as 700-1200 m.sup.2 /g, and that the polyacrylonitrile fibers as a starting material are relatively expensive.
As regards the activated carbon fibers prepared from the pitch, there has been proposed a method for the preparation thereof, for example, by spinning an optically isotropic petroleum or coal pitch into fibers, and infusibilizing the resulting fibers, followed by carbonization and activation treatment (Japanese Patent L-O-P Publn. No. 132629/1986 and 27315/1987). In this method, however, there is involved such a problem that the fiber strength of the activated carbon fibers obtained is as low as not more than 20 kg/mm.sup.2, though the activated carbon fibers are obtained from a relatively cheap starting material such as pitch and have such a large specific surface area as 1000-2000 m.sup.2 /g.
In this manner, the activated carbon fibers known in the art prepared from pitch have such a problem that they are poor in fiber strength, though they are low in the production cost thereof and are excellent in specific surface area and adsorption efficiency in comparison with the polyacrylonitrile activated carbon fibers.
Further, the carbon fibers starting from pitch are prepared by spinning petroleum pitch or coal pitch into fibers, and infusibilizing the resulting fibers, followed by carbonization and/or graphitization in an inert gas. Mechanical properties of the carbon fibers thus prepared, however, are greatly influenced by the properties of the pitch for spinning, and it is known that general purpose (GP) carbon fibers low in strength and modulus of elasticity are obtained from an optically isotropic pitch, and in the meantime high-performance (HP) carbon fibers high in strength and modulus of elasticity are obtained from a mesophase (liquid crystal) pitch containing an optically anisotropic phase.
When the carbon fibers are prepared by using the mesophase pitch, however, because the resulting carbon fibers are oriented in the axial direction of fiber at the time of spinning thereof, the fiber surfaces are liable to cleavage at the time of carbonization and/or graphitization treatment. Various researches have heretofore been made in order to solve the above-mentioned problem.
For example, Japanese Patent L-O-P Publn. No. 170528/1987 discloses a method for inhibiting occurrence of cleavage on the fiber surface of the carbon fibers, wherein a starting pitch is prepared by adding and mixing together an optically isotropic pitch and an optically anisotropic pitch so as to prepare the carbon fibers of a double structure having the surface layer consisting of the optically isotropic components and the center core consisting of the anisotropic components.
In this method, however, the pitch for spinning is prepared by simply melting and mixing together the optically isotropic pitch and the optically anisotropic pitch, and hence the optically anisotropic components are hard to disperse uniformly in the optically isotropic components, with the result that the spinnability of the pitch becomes poor, and no carbon fibers having the abovementioned double structure are obtained with great satisfaction.